Contamination of water by inorganic species such as metal ions, phosphate ions, sulfate ions, ammonium ions, metal oxyanion complexes and cyanide ions, or by acids, is a serious environmental concern in many places. For example, acid mine drainage is a common problem wherever sulfidic mine tailings are stored. Typically, acid mine drainage contains high concentrations of trace metals including toxic metals such as arsenic, cadmium, chromium, lead and others. Containment of water with a low pH and a high toxic metals load is a serious problem, because release of such water into rivers or natural aquifers could potentially cause serious environmental damage. Strategies for management of acid mine drainage water exist but they are costly, or are not completely effective, and usually involve long term monitoring and management to avoid the reestablishment of environmentally undesirable conditions caused by the influx or redevelopment of acid conditions and consequent redissolution of adsorbed and/or precipitated contaminants.
Other environmental problems resulting from contamination of water also exist. For example, algal blooms resulting from high phosphate loads in waterways or bodies of water are an increasing concern around the world. Thus, there is a need for processes and compositions for remediating contaminated water that are simple, relatively inexpensive, and effective for the removal of inorganic contaminants.
Bauxite refinery residues, commonly known as “red mud”, which have been neutralised with seawater have been shown to be capable of binding trace metals such as copper, cadmium, zinc, chromium, nickel and lead under suitable conditions.
Neutralisation of red mud is necessary because red muds are typically highly caustic, with a reaction pH of about 13.0. Consequently, they are hazardous to transport or store and storage facilities require licensing, long term monitoring and management, and final site remediation. The caustic red mud is unsuitable for most reuse applications, particularly those associated with environmental remediation, because it is unsafe to transport and apply, because it is a potential source of large quantities of sodium, and because if it were used for treatment of acidic water the presence of strong bases (particularly sodium hydroxide) creates a risk of overshooting treatment targets and replacing one set of environmental problems by another (e.g. replacing an acid problem by an alkali problem and releasing some previously bound metals that are soluble under elevated pH conditions).
The safe long term storage and management of caustic red mud is also a major problem for bauxite refineries worldwide as is the eventual rehabilitation of the storage facilities.
It is possible to neutralize red mud by adding strong acid (e.g. sulphuric) but this is an expensive management option, except where there is a surplus of waste acid. Furthermore the resulting solid material has no value as an acid neutralizing agent because most of the hydroxides and carbonates have already reacted with the added hydrogen ions, and much of the value of the material as a binding agent for trace metals and some other inorganic ions have also been lost. The caustic red mud can also be suspended in water and used to scrub potentially acid forming gases (particularly sulphur dioxide and nitrogen oxides) from industrial chimney emissions. Weak organic acids produced by decomposing vegetation (e.g. compost of mulch) can also be used to neutralize the caustic red mud during rehabilitation of red mud storage facilities, but the resulting neutralization is superficial and red mud below the treated surface can remain caustic (and potentially hazardous) for hundreds of years.
The neutralisation of red mud with seawater requires treatment of the red mud with a considerable volume of seawater relative to the volume of red mud. Accordingly there is a need for a process for neutralising red mud which is more economical and capable of being carried out in a more controlled way. Furthermore, not all bauxite refineries are close enough to the sea to be able to neutralise red mud with seawater. There is therefore a need for a process for neutralising red mud that is capable of being used at locations that are not close to the sea.
One aspect of the present invention is based on a discovery by the inventors of how seawater causes the neutralisation of red mud. This knowledge can thus be applied to enable the bauxite refinery residues to be neutralised by other means.
Although treatment of contaminated water such as acid mine drainage with neutralised red mud is capable of effecting a substantial improvement in the quality of the water, the quantities of neutralised red muds required are relatively large. Hence, there remains a need for a more economical process for treating contaminated water using neutralised red mud.
Surprisingly, the present inventors have discovered that such properties as immobilisation (including the minimisation of equilibrium repartitioning), reserve alkalinity, sludge volume reduction, and prolonged activity are enhanced and substantial reduction in the cost of treating contaminated water, such as acid mine drainage, can be achieved by treating the water in stages, in which at least one of the stages includes the use of neutralised bauxite refinery residue. The treatment also causes flocculation and the consequent reduction in the total suspended solids load in the water.